High dynamic range approach for a CMOS imager using a rolling shutter and a gated photocathode

ABSTRACT

An imaging system includes a photocathode, configured to be gated ON/OFF at a selected gating frequency, for converting photons from an object into electrons and selectively transmitting the electrons toward an imaging sensor. The imaging sensor is configured to receive the electrons and provide a rolling shutter read out of rows of pixels. The imaging sensor includes a variable well selectively set to charge the rows of pixels to a first intensity level during a first integration period and charge to a second intensity level during a second integration period. The second integration period is longer than an OFF time of the photocathode gating frequency. The first and second integration periods are equal to a frame duration of the imaging sensor, defining a number of frames per second, and the selected gating frequency of the photocathode is higher than the number of frames per second.

FIELD OF THE INVENTION

The present invention is related, in general, to CMOS image sensors.More specifically, the present invention is related to increasing thedynamic range of a CMOS image sensor when using a rolling shutter and agated photocathode.

BACKGROUND OF THE INVENTION

CMOS image sensors are used widely, for example, in digital cameras andnight vision goggle (NVG) devices. When exposed to light, the CMOS imagesensor captures an image. The image sensor typically includes a largearray of pixels that are organized into rows. There are times when thepixels in the array are not all exposed to light at the same time.Rather, the pixels are exposed sequentially, row by row. This method isknown as a rolling shutter. The exposure time for a single row of pixelsis called the exposure period. The total time required to expose andprocess the pixels in the entire array is known as the frame period.

Another method of capturing an image is known as a global shutter, or asnapshot operation. In this method, the start and end of integration forall rows in the imager is the same. Typically, at the end ofintegration, the pixel values are moved to storage capacitors to be readout while the next integration cycle begins.

One problem associated with the rolling shutter method is that theillumination level of the light source may vary over time. Thisvariation is called flicker. When exposed to flicker, an image sensormay capture the flicker as bands of contrasting brightness in the finalimage. When exposed to very bright light, the final image may beoverexposed.

FIG. 1 shows a basic three-transistor pixel circuit 101 used in priorart image sensor arrays. A transistor M1 connects a cathode (node 103)of a photodiode 125 to a voltage supply, Vdd 107. The anode ofphotodiode 125 is connected to ground. The gate of transistor M1 isconnected to a reset signal 109. Transistor M3 connects Vdd 107 toanother transistor M5. The gate of transistor M3 is connected to node103. The gate of transistor M5 is controlled by a row select signal 111,while its source is connected to a column output line 113, from whichthe output of pixel circuit 101 is read. Transistor M3 is used as asource follower to buffer photodiode 125 and prevent it from beingloaded down by column output line 113.

During operation, photodiode 125 is reset to the supply voltage Vdd 107at the beginning of an exposure period, by asserting reset signal 109and charging node 103. As photodiode 125 is exposed to incident light,it accumulates more charge and the voltage at node 103 decreases. Thevoltage across photodiode 125 is indicative of the light intensity thatphotodiode 125 has been exposed to over time. At the end of the exposureperiod, row select signal 111 is asserted to read out the values of arow of pixels in the image sensor array.

In operations using electronic image intensifiers (EI²) the photodiodeis not used. Charge is directly accumulated into the pixel capacitance,as it is “seeing” electrons not photons.

Pixel circuits are generally designed to improve pixel sensitivity underlow-light conditions. However, if lighting conditions are too bright,the photodiode accumulates too much charge and reaches saturation, atwhich point the voltage at node 103 falls to zero. Further exposure ofthe photodiode cannot be registered, because the voltage cannot fallbelow zero. As a result, the output signal of the pixel is clipped, andthe final image looks overexposed.

FIG. 2 shows two different transfer curves for the conversion gain of apixel, such as that shown in FIG. 1, assuming that its gain could bevaried. The figure plots the signal output of a pixel versus theintensity of the incident light during an exposure period. Thesaturation level of the photodiode in the pixel is indicated by dottedline 305. The line 307 is a transfer curve with one level ofsensitivity, which clips at a low light intensity level. The line 309 isanother transfer curve which provides lower sensitivity, but does notclip as early as line 307. It is desirable, therefore, to increase thedynamic range of a CMOS imager by preventing its pixels from saturatingand clipping the received light intensity.

The dynamic range of a CMOS imager is further complicated whenconsidering night vision goggle (NVG) systems. Referring to FIG. 3,there is shown an NVG system, designated generally as 30. The NVG systemincludes photocathode 31, multi-channel plate (MCP) 32 and CMOS imager33. The light, as photons, are received by photocathode 31 and convertedinto electrons. The electrons are amplified by MCP 32 and sent toelectron-sensing CMOS imager 33. The CMOS imager includes control andprocessing electronics (not shown) for providing a processed digitalvideo output to a user.

Two control signals that are pertinent to the present invention areshown functionally in FIG. 3. As shown, a gated signal turns ON/OFF thephotocathode, thereby acting as a shutter control for the photocathode.When the gated signal is ON, the photocathode permits received light topass through the photocathode and be transmitted as electrons toward theCMOS imager. When the gated signal is OFF, however, the photocathodeacts as a closed shutter and prevents light transmission to the CMOSimager.

The other control signal is referred to herein as Vresetlow, which setsa threshold voltage level in the CMOS imager, so that any lightintensity above the set threshold voltage level is clipped. Theoperation of this control signal is explained by referring to FIGS. 4and 5.

Referring first to FIG. 4, each pixel includes two integration periods,referred to herein as integration₁ and integration₂ (also referred to ast1 and t2). It will be understood that the duration of each integrationperiod may be varied. For explanation purposes, FIG. 4 shows the periodof integration₁ as 15 msec; and the period of integration₂ as 1 msec.Thus, to fully charge a pixel, an integration period of 16 msec isrequired, which includes integration₁ and integration₂.

At the pixel level of the imager, the integrated charge on the pixel hasa predetermined set threshold level that the charge cannot exceed duringthe first period of the integration time (integration₁). In the exampleshown in FIG. 4, the charge cannot exceed 3000 ADUs during the firstperiod. During the second period (integration₂), however, the Vresetlowis removed, so that the pixel is able to continue integrating, until afull charge is obtained at 4095 ADUs. It is assumed in the example thatthe full charge is 4095 ADUs and that the first integration periodcannot charge above 3000 ADUs. It will be appreciated, however, thatthese ADU levels may be different and may be set to other levels.

The above described approach is known as a variable well. At the firstintegration period (for example 15 msec), the well of the pixel cannotcharge above a set threshold (for example 3000 ADUs). During the secondintegration period (for example 1 msec), the well of the pixel ispermitted to charge up to its full well capacity of 4095 ADUs (forexample), designated as 41 in FIG. 4.

As demonstrated by curve 42 in FIG. 4, the pixel has a relatively brightinput, so it quickly reaches the set threshold of 3000 ADUs. The pixelis held at that charge until 15 msec expires. The pixel is releasedafter the 15 msec period to integrate up to its full well capacity. Thishelps the pixel in preventing saturation under high illuminationconditions and, thereby, increases the dynamic range of the incominglight that the imager may capture. This is also referred to herein as ahigh dynamic range (HDR).

A different phenomenon, however, may be seen by examining curve 43. Asshown, the pixel integrates during the first integration period. Becausethe light is not as bright, as compared to the light seen by the pixelintegrating under curve 42, the pixel never reaches the Vresetlowthreshold of 3000 ADUs. During the second integration period, the pixelis released to integrate again and continues to charge until the end ofthe frame period.

Similarly, upon examining curve 44, dark areas of an image never reachthe Vresetlow threshold (3000 ADUs, for example). The pixel continues tointegrate normally, as shown by curve 44.

Referring now to FIG. 5, there is shown an exemplary method for readingout pixel intensities during a rolling shutter operation. The exampleassumes that there are 1024 rows (also referred herein as lines) in thepixel array of the CMOS imager. As shown, the readout of frame_(N-1) andframe_(N) assumes a readout period of 16.67 msec (60 Hz image). Eachframe is gated ON/OFF by a gated signal acting as a shutter on thephotocathode. Line 1 of the pixel array is gated ON/OFF, as shown. Line2 of the pixel array is gated ON/OFF at the same time as line 1, but isread out approximately 16 μsec later due to the rolling shutteroperation. This delay is incurred by a line rate of approximately 62 kHz(1/line rate equals approximately 16 μsec). By the time line 512 is readout, during the rolling shutter, a delay of approximately 8.2 msec isincurred (512×16 μsec). By the time line 1024 is read out, during therolling shutter, a delay of approximately 16.4 msec is incurred (1024×16μsec).

Thus, as shown, each pixel integrates during frame_(N-1) and frame_(N).As the shutter rolls, each line has its integration time delayed by onerow (approximately 16 μsec). In a 1280×1024 pixel array and using a 90MHz clock to is read each pixel, it takes approximately 11.11 μsec toread each pixel. Therefore, as an example, each frame is read out inapproximately 16 msec (1280×11.11 μsec equals approximately 16 msec).

When a variable well approach is added to a night vision deviceincluding a gated photocathode, the timing interaction between one frameand the next frame becomes more significant. Furthermore, when a rollingshutter approach is used (as compared to a global shutter approach), thetiming interaction becomes even more significant. As will be explained,the present invention provides an improvement in the dynamic range of aCMOS imager, when all three of the above timing events are involved. Inother words, when the CMOS imager's performance is based on (1) the lineintegration time, (2) the timing of the variable well's break-pointduring integration, and (3) the duty cycle of the gated pulse of thephotocathode, the present invention provides an increased dynamic rangefor such a CMOS imager.

SUMMARY OF THE INVENTION

To meet this and other needs, and in view of its purposes the presentinvention provides an imaging system including a photocathode,configured to be gated ON/OFF at a selected gating frequency, forconverting photons from an object into electrons and selectivelytransmitting the electrons toward an imaging sensor. The imaging sensoris configured to receive the electrons and provide a rolling shutterread out of rows of pixels. The imaging sensor includes a variable wellselectively set to charge the rows of pixels to a first intensity levelduring a first integration period and charge to a second intensity levelduring a second integration period. The second integration period islonger than an OFF time of the photocathode gating frequency. Inaddition, the first and second integration periods are equal to a frameduration of the imaging sensor that defines a number of frames persecond, and the selected gating frequency of the photocathode is higherthan the number of frames per second.

The first intensity level is a variable set by a Vreset1 voltage, andthe second intensity level is greater than or equal to the firstintensity level. Furthermore, the second intensity level is less than orequal to a full well level of a pixel in the rows of pixels.

The rolling shutter readout provides a sequential read out of each rowof pixels during at least one of the first and second integrationperiods. Each row of pixels is read out at a frequency of the number offrames per second.

The photocathode is configured to be gated OFF with a pulsed signal,denoted as a gate_off_time. The photocathode includes at least onegate_off time per frame of the imaging sensor, denoted as onegate_pulse_per_frame. The selected gating frequency includes a timeperiod expressed as follows:

(gate_off_time/gate_pulse_per_frame) is less than the second integrationperiod.

The time period of the (gate_off_time/gate_pulse_per_frame) is less thanthe second integration period by a factor of beta, where beta is anadditional factor for providing a minimum amount of integration time.The selected gating frequency is increased to provide at least twogate_pulse_per_frame. The selected gating frequency may be at least 120Hz, and a number of frames per second of the imaging sensor may be 60Hz.

The selected gating frequency may be at least 960 Hz, and a number offrames per second of the imaging sensor may be 60 Hz.

In another embodiment of the present invention, a night vision goggle(NVG) system includes a photocathode, configured to be gated ON/OFF at aselected gating frequency, for converting photons from an object intoelectrons and selectively transmitting the electrons toward an imagingsensor. The imaging sensor is configured to receive the electrons andprovide a rolling shutter read out of rows of pixels. The imaging sensorincludes a variable well selectively set to charge the rows of pixels toa first intensity level during a first integration period and charge toa second intensity level during a second integration period. The secondintegration period is longer than an OFF time of the photocathode gatingfrequency. The first and second integration periods are equal to a frameduration of the imaging sensor, defining a number of frames per second,and the selected gating frequency of the photocathode is higher than thenumber of frames per second.

In still another embodiment of the present invention, a method ofimaging in a night vision goggle system (NVG) includes the steps of:

gating a photocathode ON/OFF at a selected gating frequency;

receiving electrons from the photocathode, by an imager using a rollingshutter;

selectively charging pixels of the imager using a first integrationperiod and a second integration period; and

controlling the second integration period, so that it is longer than anOFF time period of the selected gating frequency.

Charging the pixels is provided during a frame period, defining a numberof frames per second, and the selected gating frequency is higher thanthe number of frames per second.

The photocathode is configured to be gated OFF with a pulsed signal,denoted as a gate_off_time. The photocathode includes at least onegate_off time per frame of the imaging sensor, denoted as onegate_pulse_per_frame. The selected gating frequency includes a timeperiod expressed as follows:

(gate_off_time/gate_pulse_per_frame) is less than the second integrationperiod.

time.

The time period of the (gate_off_time/gate_pulse_per_frame) is less thanthe second integration period by a factor of beta, where beta is anadditional factor for providing a minimum amount of integration time.The selected gating frequency is increased to provide at least twogate_pulse_per_frame.

It is understood that the foregoing general description and thefollowing detailed description are exemplary, but are not restrictive,of the invention.

BRIEF DESCRIPTION OF THE FIGURES

The invention may be understood from the following detailed descriptionwhen read in connection the accompanying figures.

FIG. 1 is a conventional pixel circuit including three transistors thatmay be used in an array of an imaging sensor.

FIG. 2 shows two conversion gain curves for a pixel, such as the oneshown in FIG. 1, having two different levels of gain.

FIG. 3 is a functional diagram of a CMOS imager used in a conventionalnight vision goggle (NVG) system, including a photocathode gating pulsefor shuttering the photocathode and a voltage reset signal for setting athreshold for a variable charging well of a pixel in an array of pixels.

FIG. 4 shows three different curves for a pixel in an array of pixels,each curve depicting a different gain function during two integrationperiods.

FIG. 5 is an exemplary timing diagram for reading out pixel intensitiesin multiple rows of an imaging array, during a rolling shutter operationand a photocathode ON/OFF gating control.

FIG. 6 is an exemplary timing diagram showing charges accumulated onpixels in two different rows of an imaging array, during a rollingshutter operation and a photocathode ON/OFF gating operation.

FIG. 7 is another exemplary timing diagram showing charges accumulatedon pixels in two different rows of an imaging array, during a rollingshutter operation and a photocathode ON/OFF gating operation, in whichthe gating frequency is 1 times (1×) the number of frames per second(FPS).

FIG. 8 is yet another exemplary timing diagram showing chargesaccumulated on pixels in two different rows of an imaging array, duringa rolling shutter operation and a photocathode ON/OFF gating operation,in which the gating frequency is 2 times (2×) the number of frames persecond (FPS).

FIG. 9 is a plot of added dynamic range in dB versus a photocathode gateOFF time duration in percent, for different gating frequencies providedby the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With a global shutter readout, the timing issues in the integration ofcharge by each pixel may be easily resolved since all three of theaforementioned timing events (namely, the pixel integration time, thevariable well break-point timing during integration, and the duty cycleof the photocathode gated pulse) may be synchronized at beginning of theintegration time. With a rolling shutter CMOS device, however, simplysynchronizing the integration time, the variable break-point timing andthe duty cycle of the gated pulse is not sufficient. In fact, theinventors discovered that simple synchronization of all three events mayresult in images having the bottom half of each frame completelydestroyed.

Referring now to FIG. 6, there is shown a charge accumulated on pixelsin two different rows of a rolling shutter operation. The line 1 of thepixel array is gated ON by the line 1 gated pulse of the photocathode(shown as gate N). The line 512 is gated ON approximately 8.2 msec later(see FIG. 5) by the line 512 gated pulse of the photocathode.

While the gating of the photocathode occurs at the same time for all thelines (or rows) of each imaging frame, nevertheless, due to the rollingshutter delays between one row and another row, the integration of eachrow occurs partially during frame N and partially during frame N+1 (thisis true for all rows, except row 1 (line 1) which integrates fullyduring frame N). This is also shown in FIG. 5, by way of example, asframe N−1 and frame N. It will be appreciated, however, that FIG. 6 ispresented differently than FIG. 5. As shown in FIG. 6, the impact of therolling shutter and the photocathode gating pulse on each line of pixelsmay be seen more clearly, because there is shown a synchronizedintegration starting time for each row of pixels, during onephotocathode ON/OFF gating period. Thus, line 1 integrates completelyduring the gate N period, whereas line 512 integrates partially duringthe gate N period and partially during the gate N+1 period (compare thiswith the gating shown FIG. 5).

Still referring to FIG. 6, the frame integration period is divided intotwo integration periods (or regions), namely, integration time t₁ andintegration time t₂. The time spent integrating in each region iscontrollable. In the example shown, integration time t₁ hasapproximately a 15 msec duration and integration time t₂ hasapproximately a 1 msec duration. This provides a total integration timefor each line of 16 msec (actually 16.67 msec for a 60 Hz imager).

The clamping voltage (Vreset1) is also a controlled parameter whichadjusts the voltage a pixel is clamped to during the first integrationperiod, t₁. At the end of the first integration period, the pixel isreleased to integrate again during the second integration period, t₂.

Areas of uneven exposures (or offsets) are created in the imagerdepending on how the photocathode OFF times align with the pixel'sintegration regions (periods t₁ and t₂). As shown in FIG. 6, a pixel inline 1 is clamped at Vreset1, whereas a pixel in line 512 never clamps.The line 1 pixel charges as shown by curve 61. The pixel in line 1charges until clamped at Vreset1. After start of the second integrationperiod t₂, the pixel in line 1 is released and continues charging, untilshuttered by the photocathode gate (shown as region 64.)

The pixel in line 512, however, never clamps. As shown, by curve 62, thepixel in line 512 charges until it is shuttered by the OFF time of thephotocathode gate (region 63). At the next ON time of the photocathodegate, the pixel in line 512 continues charging again and is neverclamped by the Vreset1 voltage. The pixel continues to charge, while itis in the second integration period, t₂, until it reaches the end of theframe integration time (period t₁ plus period t₂).

Because the pixel in line 512 never clamps, the pixel charges to ahigher ADU level than the pixel in line 1, even though both lines ofpixels experience an equivalent input flux. Notice that curve 62 reachesa higher ADU voltage than curve 61.

Referring next to FIG. 7, there is shown curves 71 and 72 which depict,respectively, the integration of line 1 (row 1) pixels and line 512 (row512) pixels. Also shown are two integration periods, namely, firstintegration period t₁ and second integration period t₂. It will beappreciated that FIG. 7 is similar to FIG. 6, except for the startinglocation of the second integration period t₂ (indicated by a heavy blackdot). In FIG. 6, the starting location of the t₂ period falls within thegate ON time of the line 1 pixels. In FIG. 7, however, the startinglocation of the t₂ period falls within the gate OFF time of the line 1pixels (shown as region 74).

Referring to FIG. 7, the line 1 pixels integrate according to curve 71.As shown, the line 1 pixels integrate during the entire photocathodegate ON time, integrating through region 73. The line 1 pixels integrateuntil reaching the pre-selected threshold Vreset1. The line 1 pixelsstop integrating until reaching the second integration period t₂. Theline 1 pixels, nevertheless, do not start integrating again, because thephotocathode gate is now OFF. Accordingly, the line 1 pixels neverexperience the second integration period t₂, because the gate is OFF.

The line 512 pixels, on the other hand, integrate according to curve 72.As shown, the line 512 pixels integrate until the gate is turned OFFduring region 73. After the gate is ON again, the line 512 pixels startintegrating again, and continue integrating until the end of the frameperiod (60 HZ imager, as an example). Thus, the line 512 pixels reach ahigher ADU than the line 1 pixels.

The line 1 pixels, thus, have no resolvable contrast above the clampingthreshold of Vreset1. Since the line 1 pixels are clamped at the Vreset1level, there is no additional voltage charge (or contrast) which may beused to correct the error during the second integration period. Having aphotocathode gated frequency of 1 times (1×) the number of frames persecond (FPS), as illustrated in FIG. 7, is a noticeable disadvantage. Aswill be explained, by increasing the gate frequency to a point where thephotocathode gated pulse OFF period is shorter than the secondintegration period t₂, a contrast above the Vreset1 voltage level may beachieved by the present invention.

In summary, the present invention allows for using a variable well, aphotocathode gated pulse and a rolling shutter, without the difficultiesdescribed above with respect to FIG. 7. Furthermore, the presentinvention provides a correction method for the non-uniformity induced ina resulting image by the timing interactions among the threeaforementioned components. In addition, the present invention providestiming relationships for the imaging system that may be maintained forany possible correction method.

In order to obtain an image that is correctable in a post processingstep, the timing of the three components of the system is controlled bythe present invention, so that every row experiences at least someportion of the integration time during the second integration period ina variable well scheme. The manner in which the present inventionaccomplishes this is by increasing the gating frequency of thephotocathode. This is described further below by reference to thegraphic curves shown in FIG. 8. Of importance in FIG. 8 is making surethat every row has a chance to integrate above a clamping voltage presetby a variable well approach.

The inventors discovered that the gating frequency of the photocathoderequires a relationship with a minimum time in which the imager mayspend during the second integration period of a variable well approach.This may be described by the following relationship:(gate_off_time/gate_pulse_per_frame)+β<T_int2,

where β is an additional factor that is tied to a grayscale resolutionthat may be achieved during the second integration period of thevariable well approach by including a minimum amount of integration timefor the second integration period.

The shorter the time that the imager spends in the second integrationperiod of a variable well approach, the higher the input light/currentthat the imager may see without saturating the imager. This results in ahigher dynamic range. Thus, in order to shorten the second integrationperiod, using the relationship above, the gating frequency is increasedby the present invention. FIG. 9 provides a graphic presentation thatshows the improved relationship for an exemplary imager.

As shown in FIG. 9, dynamic range is increased as the gating frequencyof the photocathode is also increased. For example, for a 60 Hzphotocathode gating frequency and a 50% photocathode gating duty cycle,no increase in dynamic range is experienced by the present invention.However, for the same 60 Hz gating frequency, as the gating duty cycleis increased to 100% (or as shown in FIG. 9, the gating OFF time isdecreased to zero), the dynamic range of the imager increases by as muchas 30 dB.

By increasing the gating factor as the duty cycle deceases, the secondintegration period may be kept short, thereby obtaining extra dynamicrange (DR). At a gating frequency of 960 Hz (16×60 FPS), a DR increaseof 23 dB may be maintained down to at least 0.04% duty cycle (a minimumpossible gate duty cycle). Given complications of increased gatingfrequency (such as increased EMI and increased power usage), a 16×gating frequency may be a good choice. At a gating frequency of 3.84 kHz(64×60 FPS), a DR increase of over 30 dB may be maintained down to atleast 0.04% duty cycle. This gating frequency may also be a good choice.

It will be appreciated that pixel integration during the secondintegration period is the feature that provides the DR increase. Byincreasing the photocathode gating frequency to a point in whichindividual gating pulses are shorter than the second integration period,a contrast above the Vreset1 threshold may be achieved. This isillustrated, by way of example, in FIG. 8.

As shown in FIG. 8, the gating frequency is increased to 2 times (2×)the gating frequency of a 60 FPS imager (gating frequency of 120 Hz).Recall that FIG. 7 illustrated a gating frequency of 1× (or 60 Hz). InFIG. 8, the line 1 gate has twice the rate of the line 1 gate shown inFIG. 7; and the line 512 gate also has twice the rate of the line 512gate shown in FIG. 7. As a consequence of increasing the gatingfrequency to twice that of the gating frequency shown in FIG. 7, the DRis increased.

The integration of a pixel in line 1 is shown by curve 81, whereas theintegration of a pixel in line 512 is shown by curve 82. The line 1pixel continues to charge through region 83 and stops to charge inregion 84. The line 1 pixel continues to charge again after passingregion 84. The line 1 pixel charges until clamped by the Vreset1threshold voltage, and continues to be clamped until the pixel reachesthe end of the first integration period t₁.

At the start of the second integration period t₂, however, the line 1pixel is released to continue charging. The line 1 pixel continues tocharge above the Vreset1 threshold level, until stopped again by thesecond OFF period of the photocathode gated frequency (2×), shown asregion 86. Thus, the line 1 pixel experiences some portion ofintegration time during the second integration period.

The integration of the line 512 pixels are shown by curve 82. The line512 pixels continue charging until stopped by the first gate OFF periodin region 83. After the gate is ON again (after passing region 83), theline 512 pixels continue charging until clamped by the Vreset1 thresholdlevel. The line 512 pixels continue to be clamped through region 85until arriving at the end of the first integration period. The line 512pixels begin charging again, during the second integration period t₂,because the line 512 gate is ON, and do not stop charging until reachingthe end of the frame. Thus, the line 512 pixels experience some portionof integration time during the second integration period (the portion islonger than the portion allocated for line 1 pixels).

By comparing FIG. 8 with FIG. 7, it will be appreciated that integrationduring the second integration period t2 is the driver for increasing thedynamic range for the imager. The gate OFF period, thus, is selected bythe present invention to be smaller than the second integration periodt2. In this manner, every row of pixels experiences some integrationduring a portion of the second integration period. In FIG. 7, the line 1pixels never experience the second integration period, because thephotocathode gate is OFF. In FIG. 8, however, the line 1 pixels reach acharge level above the selected Vreset1 voltage.

Any error in contrast between the peak voltage of curve 81 and the peakvoltage of curve 82 may be corrected by using the following reasoning:

(1) The relationship between gating and integration is known.

(2) Based on the integration time for a row and the resultant ADU outputlevel, the input current to the pixel may be calculated.

(3) For pixels that never reach a clamping voltage, the input currentmay be calculated exactly.

(4) For pixels above the clamping voltage, there is ambiguity as towhether the pixel reached the clamping voltage during the firstintegration period, or the second integration period was required toreach the threshold. This does result in some residual error.

(5) Assuming the input current is constant during the integrationperiod, the current may be calculated by varying the time that the pixelis assumed to be clamped, in order to maximize the calculated inputcurrent.

(6) Provided these conditions are met, the correction technique is veryeffective. As the time in the second integration period is reduced,however, the number of gray scales may be reduced and there may be apotential for SNR issues where noise is amplified or creates errorsduring post processing.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

What is claimed:
 1. An imaging system comprising a photocathode, configured to be gated ON/OFF at a selected gating frequency, for converting photons from an object into electrons and selectively transmitting the electrons toward an imaging sensor, the imaging sensor, configured to receive the electrons and provide a rolling shutter read out of rows of pixels, and the imaging sensor including a variable well selectively set to charge the rows of pixels to a first intensity level during a first integration period and charge to a second intensity level during a second integration period, wherein the second integration period is longer than an OFF time of the photocathode gating frequency, the photocathode is configured to be gated OFF with a pulsed signal, denoted as a gate_off_time, the photocathode includes at least one gate_off time per frame of the imaging sensor, denoted as one gate_pulse_per_frame, and the selected gating frequency includes a time period expressed as follows: (gate_off_time/gate_pulse_per_frame) is less than the second integration period.
 2. The imaging system of claim 1 wherein the first and second integration periods are equal to a frame duration of the imaging sensor, defining a number of frames per second, and the selected gating frequency of the photocathode is higher than the number of frames per second.
 3. The imaging system of claim 2 wherein the first intensity level is a variable set by a Vreset1 voltage, and the second intensity level is greater than or equal to the first intensity level.
 4. The imaging system of claim 3 wherein the second intensity level is less than or equal to a full well level of a pixel in the rows of pixels.
 5. The imaging system of claim 2 wherein the rolling shutter readout provides a sequential read out of each row of pixels during at least one of the first and second integration periods, and each row of pixels is read out at a frequency of the number of frames per second.
 6. The imaging system of claim 1 wherein the time period of the (gate_off_time/gate_pulse_per_frame) is less than the second integration period by a factor of beta, where beta is an additional factor for providing a minimum amount of integration time.
 7. The imaging system of claim 1 wherein the selected gating frequency is increased to provide at least two gate_pulse_per_frame.
 8. The imaging system of claim 7 wherein the selected gating frequency is at least 120 Hz, and a number of frames per second of the imaging sensor is 60 Hz.
 9. The imaging system of claim 7 wherein the selected gating frequency is at least 960 Hz, and a number of frames per second of the imaging sensor is 60 Hz.
 10. A method of imaging comprising the steps of: gating a photocathode ON/OFF at a selected gating frequency; receiving electrons from the photocathode, by an imager using a rolling shutter; selectively charging pixels of the imager using a first integration period and a second integration period; and controlling the second integration period, so that it is longer than an OFF time period of the selected gating frequency, wherein the photocathode is configured to be gated OFF with a pulsed signal, denoted as a gate_off_time, the photocathode includes at least one gate_off time per frame of the imaging sensor, denoted as one gate_pulse_per_frame, and the selected gating frequency includes a time period expressed as follows: (gate_off_time/gate_pulse_per_frame) is less than the second integration period.
 11. The method of claim 10 wherein charging the pixels is provided during a frame period, defining a number of frames per second, and the selected gating frequency is higher than the number of frames per second.
 12. The method of claim 10 wherein the time period of the (gate_off_time/gate_pulse_per_frame) is less than the second integration period by a factor of beta, where beta is an additional factor for providing a minimum amount of integration time.
 13. The method of claim 10 wherein the selected gating frequency is increased to provide at least two gate_pulse_per_frame. 